1. Field of the Invention
The present invention relates to apparatus for processing the signals generated by a multioscillator. More particularly, this invention pertains to an output system of optical and electronic elements for providing separation of components of light beams and processing signals derived from the separated components for rotation rate measurement and cavity length control.
2. Description of the Prior Art
Laser gyroscopes employ two or more waves of electromagnetic energy that counterpropagate in a laser medium. Rotation of the gyroscope, induced by that of a platform to which it is strapped, will produce a difference in the round-trip times of the oppositely-propagating waves. Such difference depends upon the rate and amount of such rotation, and may be output as a frequency difference. In a two wave planar cavity system, a lock-in phenomenon, yielding no such frequency difference, may occur at low rates of rotation. Lock-in is believed to result from coupling between waves occasioned by factors such as backscattering of wave energy from elements within the optical path or cavity including, for example, mirrors or a Faraday rotator.
Early attempts to solve the lock-in problem for planar cavities have included separate all-mechanical and all-optical biasing of the counterpropagating beams away from the zero output frequency level. All-optical biasing involves the use of magneto-optic mirrors to subject the counterpropagating beams to different delay times, while all-mechanical biasing involves physical dithering (i.e. rotational oscillation) of the gyro block.
Each of the above-identified approaches possesses recognized shortcomings. For example, optical biasing that separates the lasing beams sufficiently in frequency to avoid lock-in can produce a frequency difference that is very large in comparison to the frequency difference caused by rotation. As a result, a small amount of drift can effectively mask the desired, measured signal output. All-mechanical biasing introduces the complexities of vibrational noise suppression.
An alternative approach to solving the lock-in problem involves rotation sensors that employ more than one pair of counterpropagating beams. A beam propagates in the clockwise direction about a non-planar cavity that includes left and right circularly-polarized modes, while another beam that includes right and left circularly polarized modes propagates in the counter-clockwise direction. The four modes are created by the combined effects of cavity design (out-of-planeness) and optical biasing (e.g., Faraday rotator). In particular, so-called reciprocal splitting of right and left-circularly polarized lasing modes results from cavity nonplanarity. The magnitude of the frequency splitting is a function of the degree of non-planarity. Non-reciprocal splitting in frequency between modes of the same polarization propagating in opposite directions about the lasing cavity is produced by the introduction of, for example, a glass Faraday rotator into the cavity. Alternatively, such nonreciprocal splitting can be produced by the imposition of a magnetic field upon the plasma.
FIG. 1 is a gain vs. frequency plot illustrating the four modes within a single leg of a multioscillator. (Four consecutive legs are required to form the required out-of-plane lasing cavity. The helicities of the beams that propagate through the cavity reverse as the beams are reflected by corner mirrors into adjacent cavity legs.) The modes, shown in FIG. 1 superimposed upon the gain vs. frequency curve, are grouped into a pair of left circularly polarized modes (LCP) and a pair of right circularly polarized modes (RCP). The two pairs are located at either side of gain center. One mode of each of the above-identified pairs propagates clockwise ("c") and one propagates counterclockwise ("a") within the non-planar multioscillator cavity. The relationship of the four above-described modes provides the information required to (1) determine rotation rate and direction and (2) tune the lasing cavity on a continual basis.
The output of a four mode rotation sensing device is preferably a digital number or other signal representing the total amount of rotation experienced during a predetermined time period. Alternatively, a digital number or other signal may represent the present rate of rotation of the gyroscope. The rate of rotation is computed in accordance with the formula: EQU .OMEGA.=L.lambda./8A (.omega..sub.4 .omega..sub.3)-(.omega..sub.2 -.omega..sub.1)! (1)
where .OMEGA. is the rate of rotation about the sensitive axis of the gyro, L is the total path length, A is the area effectively enclosed within the path and .lambda. is the wavelength of the waves propagating within the laser gyroscope cavity. The amount of rotation is found by integrating the above equation with respect to time.
In accordance with the above, it is therefore essential to determine the characteristics of each of the four beams. However, this task is complicated by the properties of the required partially-transmissive output mirror through which the rotation-affected optical beams are transmitted. Such mirrors routinely attenuate the s and p components of obliquely-incident light beams to vastly differing degrees. As a result, the ability to distinguish the various modes after transmission of the counterpropagating beams through the output mirror is significantly complicated. More particularly, commonly used output mirrors cause a significantly greater reduction in the s component than in the p component of the electromagnetic field associated with each of the beams. As a consequence, upon transmission of each of the four beams into an output prism, each beam is characterized by a nearly-linear electromagnetic field polarization rendering the polarizations of the four beams nearly-indistinguishable.
The above-stated problem has been addressed by both all-optical and all-electronic approaches. U.S. Pat. No. 4,449,824 of Matthews entitled "Laser Gyro Output Optics Structure" discloses all-optical elements for separating the various beams. There, the undesired polarization components are eliminated from the various HET signals. As a result, a substantial amount of useful signal is also lost, rendering the system output vulnerable to noise. U.S. Pat. No. 5,116,132 of Mitchell et al. entitled "Ring Laser Gyroscope Output Optics Detection System" discloses all-electronic signal separation circuitry that imposes a ninety-degree phase shift to one set of heterodyne signals in order to extract the Sagnac effect-modulated Faraday frequency signals. However, the ability of the circuit to extract the Sagnac effect-modulated Faraday frequency signals is severely reduced at high rotation rates. At very low rates, lock-in, similar to that observed in two mode devices, can occur as the cavity length control signal derived from the Sagnac effect-modulated Faraday frequency signals is modulated at the slow gyro beat frequency.